1. Field of the Invention
The present invention relates to a braking force control system for an automotive vehicle, and more particularly to the braking force control system which is adapted to maintain a characteristic of a steering control for tracing a vehicle's path against a rapid yaw rate variation of the vehicle, by applying a braking force to at least one of the wheels of the vehicle irrespective of depression of a brake pedal.
2. Description of the Related Arts
Recently, a vehicle is provided with a vehicle motion control system for controlling the braking force applied to the vehicle to perform an anti-skid control, a traction control, a front-rear braking force distribution control, and etc. In the U.S. Pat. No. 4,898,431, for example, an apparatus for controlling braking force through the use of a brake controlling system which compensates for the influence of lateral forces on the vehicle. The apparatus is constituted so as to control the braking force applied to the vehicle by the braking force control system in response to a comparison of a desired yaw rate with an actual yaw rate, thereby to improve the vehicle stability during the course of vehicle motion such as cornering.
In general, "oversteer" and "understeer" are used to indicate a vehicle steering characteristic. When the oversteer is excessive during vehicle motion such as cornering, the rear wheels of the vehicle have a tendency to slip excessively in the lateral direction to cause a decrease in turning radius of the vehicle. The oversteer occurs when a cornering force CFf of the front wheels largely exceeds a cornering force CFr of the rear wheels (i.e., CFf&gt;&gt;CFr). As shown in FIG. 20, when a vehicle VL is undergoing a cornering maneuver along a curve of a turning radius R, for example, a lateral acceleration Gy which is normal to the vehicle's path of motion is calculated in accordance with an equation of Gy=V.sup.2 /R, where "V" corresponds to a vehicle speed, and a total CFo of the cornering force is calculated in accordance with the following equation: EQU CFo=.SIGMA.CF=m.multidot.Gy
where "m" corresponds to a mass of the vehicle VL. Accordingly, in the case where the sum of the cornering force CFf and the cornering force CFr is larger than the total cornering force CFo for the vehicle's cornering maneuver along the curve of the turning radius R (i.e., CFo&lt;CFf+CFr), and the cornering force CFf of the front wheels largely exceeds the cornering force CFr of the rear wheels (i.e., CFf &gt;&gt;CFr), i.e., the oversteer is excessive, then the vehicle VL will be forced to turn in a direction toward the inside of the curve in the vehicle's path to cause a reduce in turning radius of the vehicle VL as shown in FIG. 20.
When the understeer is excessive during cornering, the lateral slip of the vehicle will be increased, the vehicle VL will be forced to turn in a direction toward the outside of the curve in the vehicle's path to cause an increase in turning radius of the vehicle VL as shown in FIG. 21. Thus, the excessive understeer occurs when the cornering force CFf of the front wheels is almost equal to the cornering force CFr of the rear wheels so as to be balanced with each other, or the latter is slightly larger than the former (i.e., CFf&lt;CFr), and when the sum of the cornering force CFf and the cornering force CFr is smaller than the total cornering force CFo which is required for the vehicle's cornering maneuver along the curve of turning radius R (i.e., CFo&gt;CFf+CFr), then the vehicle VL will be forced to turn in the direction toward the outside of the curve in the vehicle's path thereby to increase the turning radius R.
The excessive oversteer is determined on the basis of a vehicle side slip angle or vehicle slip angle .beta. and a vehicle slip angular velocity D.beta., for example. When it is determined that the excessive oversteer occurs during cornering, a braking force will be applied to a front wheel located on the outside of the curve in the vehicle's path, for example, to produce a moment for forcing the vehicle to turn in the direction toward the outside of the curve, i.e., an outwardly oriented moment, in accordance with an oversteer restraining control, which may be called as a vehicle stability control. On the other hand, the excessive understeer is determined on the basis of a difference between a desired lateral acceleration and an actual lateral acceleration, or a difference between a desired yaw rate and an actual yaw rate, for example. When it is determined that the excessive understeer occurs while a rear-drive vehicle is undergoing a cornering maneuver, for example, the braking force will be applied to a front wheel located on the outside of the curve and applied to both of the rear wheels to produce a moment for forcing the vehicle to turn in the direction toward the inside of the curve, i.e., an inwardly oriented moment, in accordance with an understeer restraining control, which may be called as a course trace performance control. The above-described oversteer restraining control and understeer restraining control as a whole may be called as a steering control by braking.
In the U.S. Pat. No. 5,494,345, there is disclosed a brake control apparatus for normalizing an abnormal behavior of a vehicle by applying a braking force to each wheel. It is proposed in that publication to introduce a feedforward control to normalize the abnormal behavior of the vehicle quickly and accurately. The apparatus includes feedforward control means for producing a feedforward control signal in accordance with a target slip ratio, and mixing means for mixing the feedforward control signal with a feedback control signal indicative of a difference between the target slip ratio and the actual slip ratio and for controlling the hydraulic brake control system in accordance with the mixed control signal.
According to the apparatus disclosed in the U.S. Pat. No. 5,494,345, however, when a vehicle moves from a road of a relatively low coefficient of friction to a road of a relatively high coefficient of friction in the vehicle cornering maneuver, for example, its yaw rate will be varied rapidly to deteriorate the characteristic of the steering control for tracing the vehicle's path (abbreviated herein as a tracing control), especially in the case where the wheel located on the inside of the curve is placed on the road of the high coefficient of friction, while the wheel located on the outside of the curve is placed on the road of the low coefficient of friction. As a result, the vehicle tends to depart from the vehicle's path which is determined in accordance with the steering angle at that time. According to a conventional vehicle which is not provided with the braking force control system, nor the motion control system, if the vehicle speed is reduced, when one of its front wheels is placed on the road of the relatively high coefficient of friction, and the other of the front wheels is placed on the road of the relatively low coefficient of friction, the tracing control characteristic will be deteriorated.